Recent years have shown an increase in the number of swimming pool installations, especially those in the back yards of homes. These pools present a safety hazard, especially in the case of small children who are unable to swim and who may play in the area adjacent to the pool. Regardless of the safety precautions that are maintained, instances still arise when play may go unsupervised and there always is a chance that a child may fall into the pool with the everpresent danger of being drowned.
The prior art swimming pool alarm systems have generally been of two types, namely, those having electrical contacts which are made or broken as a result of physical movement caused by surface wave action of the water in the pool, and those employing transducers. The present invention is of the second type.
One example of a transducer-type swimming pool alarm system is described in U.S. Pat. No. 4,187,502. As stated in that patent, representative of the electrical contact type of alarm systems may be found in U.S. Pat. Nos. 4,017,842; 3,778,803 and 3,504,145.
Another transducer system is described in U.S. Pat. No. 3,810,146 in which a transducer is mounted in the wall of a swimming pool and is responsive to ultrasonic signals from special transmitters which are worn by children, or others who might inadvertently fall into the pool. Such a system, however, is limited in that it would fail to detect an unequipped person.
In U.S. Pat. No. 3,969,712, a transducer is mounted on the underside of a floating housing. The housing contains circuitry to filter out lower frequency signals generated by the transducers, to integrate the signals and to activate an alarm when a particular threshold is reached. Such a system, however, is susceptible to surface wave action and requires that a relatively large area of the underside of the housing be sealed against water.
In accordance with the teachings of the present invention, a swimming pool alarm system is provided which overcomes the problems encountered in the prior art systems. The transducer type alarm system of the present invention is unresponsive to surface wave action and physical contact with objects such as wind blown debris, and physical contact with the sides of the pool. The system of the invention exhibits omnidirectional response to underwater pressure waves and detects with high accuracy the entrance of objects into the pool. The swimming pool alarm system of the present invention utilizes a piezoelectric element as the sensing transducer. The piezoelectric element produces a voltage in response to a change in stress applied thereto. A pair of electrodes are mounted on opposite sides of the transducer element for sensing the voltage. The transducer is provided with actuator means in the form of a diaphragm for transmitting pressure to the piezoelectric element to produce stress in the element so that upon an increase in pressure the piezoelectric element is stressed to produce a voltage which when it exceeds a predetermined threshold, activates an electric alarm.